Expression of the repressible acid phosphatase (rAPase) gene, PHO5, of Saccharomyces cerevisiae is repressed by a certain level of inorganic phosphate (Pi) in the medium and is derepressed when the Pi concentration is lowered. The Pi signals are conveyed to PHO5 by a regulatory system consisting of proteins coded for by the PHO2, PHO4, PHO80 and PHO81 genes. We have found that the transcription of PHO81 is regulated by Pi through the PHO regulatory system. Increasing the dosage of PHO4 and PHO81 by ligating each gene to YEp13 gives rise to, respectively, considerable and weak synthesis of rAPase by cultivation of the transformants in high-Pi medium; but in low-Pi medium, increased dosage of PHO4 stimulates the rAPase synthesis significantly, whereas PHO81 has no effect. Increased dosage of PHO2 stimulates rAPase synthesis considerably in low-Pi but not in high-Pi. A coordinate increase of PHO80 cancels the dosage effect of PHO4, but not that of PHO81. Coordinate increases of PHO80 and PHO2 give rise to the same phenotype as an increased dosage of PHO80 alone. The level of the PHO4 protein was found to be the limiting factor of the rAPase synthesis and the copy number of the PHO5 gene not to be. These facts accord with the idea that the PHO80 protein transmits the Pi signals to the PHO5 gene via the PHO4 protein, whereas the PHO2 protein does not have a direct function in the signal transmission.

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